1. Field of the Invention
The invention relates to a vehicle head lamp that is able to irradiate OHS light that illuminates an overhead sign (hereinafter, also referred to as “OHS”) that is a sign located forward and upward of a vehicle.
2. Description of the Related Art
There is proposed a head lamp that is configured to illuminate an OHS in a low beam light distribution as a head lamp for a vehicle, such as an automobile. Japanese Patent Application Publication No. 2007-179994 (JP-A-2007-179994) describes that a light-blocking shade for forming a low beam light distribution partially has an opening and then light passing through the opening is used as OHS light. FIG. 8 shows a head lamp that is described in JP-A-2007-179994. The head lamp reflects light, emitted from a light source 4, on a reflection surface of a reflector 1 having a rotational ellipsoidal shape, blocks part of the reflected light with a shade 6B, condenses non-blocked light with a condenser lens 3 supported by a holder 2 and outputs the non-blocked light forward of the lamp to illuminate a front area of a host vehicle. The shade 6B is formed of a main shade 61 and a sub-shade 62. The main shade 61 is used to block light irradiated upward in order to obtain a low beam light distribution. The sub-shade 62 is used to form a cut line of the low beam light distribution. The main shade 61 is substantially vertical with respect to a lamp optical axis Lx. The sub-shade 62 is arranged so as to be inclined at a desired inclination angle θ0 with respect to the lamp optical axis Lx. The main shade 61 has an opening 613 below a light transmission window 611. The main shade 61 allows the opening 613 to pass part of light reflected by the reflector 1 to thereby generate light irradiated upward, and then irradiates the light as OHS light.
In the head lamp shown in FIG. 8, light reflected by an upper reflection area of the reflection surface of the reflector 1 above the lamp optical axis Lx and light reflected by a lower reflection area of the reflection surface of the reflector 1 below the lamp optical axis Lx are not blocked by the shade 6B, but become light that contributes to a low beam light distribution. Part of light reflected by a portion of the lower reflection area adjacent to the front side is blocked by the shade, and another part of light passes through the opening 613 to become OHS light. Thus, substantially all the surface of the upper reflection area is formed as an effective reflection area Su1. On the other hand, for the lower reflection area, when a position PO at which an imaginary extension plane Z0 extended rearward along an inclined plane of the sub-shade 62 intersects with the reflection surface of the reflector 1 is obtained, light reflected by a reflection area rearward of the lamp with respect to the intersection position P0 within the lower reflection area is not blocked by the shade 6B but becomes light that contributes to a low beam light distribution. This area is an effective reflection area Su2. In addition, part of light reflected by a portion of the area on a front side of the effective reflection area Su2 is blocked by the shade 6B, and another part of the reflected light passes through the opening 613 to become OHS light. The area that reflects light that becomes OHS light is an OHS reflection area So. The OHS reflection area So is designed so as to be different in surface shape from the effective reflection area Su2 so that reflected light efficiently passes through the opening 613. Note that an area Sn is an ineffective reflection area that does not contribute to a light distribution.
In recent years, there is proposed a head lamp that is intended to reduce electric power consumed by an automobile. A head lamp of this type is required to increase an effective utilization efficiency at which light emitted from a light source is effectively utilized as illumination light, and it is conceivable that the area of the effective reflection area of the reflector is expanded as one of measures for the above request in order to increase the amount of light reflected by the reflector. That is, in the case of the head lamp shown in FIG. 8, when an inclination angle θ0 of the sub-shade 62 with respect to the lamp optical axis Lx is increased as much as possible, the intersection position P0 of the imaginary extension plane Z0 of the sub-shade 62 and the reflection surface of the reflector 1 shifts forward, so the effective reflection area Su2 may be expanded. Therefore, light blocked by the shade 6B is reduced. Thus, it is possible to obtain a low beam light distribution that has a high effective utilization efficiency of light emitted from the light source 4.
However, if the inclination angle θ0 is simply increased as in the case of a sub-shade 62X shown by the imaginary line in FIG. 8, the OHS reflection area So shifts forward accordingly, so an inclination angle of light, which is reflected by the OHS reflection area So, passes between the main shade 61 and the sub-shade 62X and then passes through the opening 613 of the main shade 61, with respect to the lamp optical axis Lx also increases. If the inclination angle θ0 exceeds a certain angle, light that passes through the opening 613 does not enter the condenser lens 3 as in the case of a light beam La shown in FIG. 8, and it is difficult for the light to function as OHS light. In order for the light passing through the opening 613 to pass through the condenser lens 3, the inclination angle θ0 of the sub-shade 62 needs to be limited to an angle corresponding to the lens aperture and lens focal length of the condenser lens 3. This also limits an increase in the effective reflection area Su2 of the reflector 1.